[Metrics] Fix/4237 auc unstable reorder

[ddrevicky]

What does this PR do?

Deprecates the reorder parameter to the auc metric because of torch.argsort being unstable.

Fixes #4237

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[SkafteNicki]

Please also add note at argument:
https://github.com/PyTorchLightning/pytorch-lightning/blob/d0c96b195f5a22f454c9d73ce595e7911b9dafc1/pytorch_lightning/metrics/functional/classification.py#L808

[codecov]

Codecov Report

Merging #4281 into master will increase coverage by 1%.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #4281    +/-   ##
=======================================
+ Coverage      92%     93%    +1%     
=======================================
  Files         111     111            
  Lines        8123    8011   -112     
=======================================
- Hits         7482    7448    -34     
+ Misses        641     563    -78     

[tchaton]

Great catch ! I didn't know argsort was unstable.

[SkafteNicki]

@ddrevicky could you please solve the pep8 issue:
./pytorch_lightning/metrics/functional/classification.py:841: [F541] f-string is missing placeholders

[mergify]

This pull request is now in conflict... :(